An integrated circuit (IC) connected to external terminals is vulnerable to electrostatic discharge (ESD) pulses from the operating environment and peripherals such as human bodies or machines. An ESD event may produce high current or high voltage pulses within a few nanoseconds, leading to device degradation or damage. In order to protect the vulnerable IC from ESD damaging pulses, an ESD protection circuit has to be connected to the external terminals of main IC elements.
Compound semiconductor devices have been widely used in the radio frequency (RF) circuits market. For example, compound semiconductor high electron mobility transistor (HEMT) switches have been widely applied in 3G cell phone applications in recent year for their high performance in RF range. However, the lack of good ESD protection devices has become a major drawback in HEMT switch applications. Conventionally, ESD protection circuits are made of series-connected diodes. A single enhancement-mode FET (E-FET) with the gate connected to the source with a resistor can act as a diode with different turn-on voltage for forward (Von—forward) and reverse bias voltage (Von—reverse), as shown in FIG. 9. The E-FET with a resistor connecting between its gate and source can be used as an ESD protection device. To meet the requirement of the circuit to be protected, the overall forward and reverse turn-on voltage of the diode can be modified by connecting the E-FET diodes in series in the same or opposite direction. By applying such an ESD protection device in a monolithic microwave and millimeter wave circuit comprising compound semiconductor E-FETs, the ESD protection device and other circuit elements can be formed on the same epitaxial structure. The problem of such an ESD protection device is that it often creates nonlinearity when the circuit is under an RF operation, particularly when the RF power is high. This results in the generation of harmonic distortions (HDs), intermodulation distortion (IMD), etc. The nonlinearity is induced because the gate voltage of the E-FET with respect to the source is near zero volts. Since the pinch-off voltage of the E-FET is usually small, the RF signal applied to the parasitic capacitance across the gate and the source (Cgs) and across the gate and the drain (Cgd) experiences the voltage-dependent capacitance, which is large in the sub-threshold voltage.